usb: Fix test dependency on ext4write command

usb: Fix test dependency on ext4write command

The tests test_usb_ext4load_ext4write, test_usb_ext2load and
test_usb_load depend on the command ext4write being present not just
the feature of being able to write to an ext4 file system. So update
their dependencies to include the ext4write command itself. This will
prevent spurious test failures when running the USB tests against USB
storage with an ext2/ext4 partition but no ext4write command available.

Fixes: 1c5b6edad381 ("test/py: usb: Add tests for USB device")
Signed-off-by: Andrew Goodbody <andrew.goodbody@linaro.org>
Reviewed-by: Love Kumar <love.kumar@amd.com>